Current views of the organization of lipids and the processes which lead to the formation of lipid structures in membranes have been examined by a rigorous analysis of the phase relations of phospholipid dispersions in water. A new equilibrium surface phase has been found for lecithins: the bimolecular surface state. It is formed by adsorption and only in the presence of bulk lecithin in the dispersion. This bilayer state occurs over a narrow (5-10 degrees) temperature range and at a temperature which is characteristic of the lecithin. Membranes containing lecithin are almost universally believed to undergo a liquid condensed (LC)-liquid expanded (LE) phase transition. However, we have been able to demonstrate that lecithin does not form the LC state and therefore this transformation probably does not exist in cell membranes. Our studies indicate that one of the phase transitions is likely to involve a monolayer bilayer equilibrium in which both phases are in the liquid expanded state.